1. Field of the Invention
The present invention relates to a conductivity meter for measuring a conductivity of a solution and, more particularly, to an improved electrode and a method of producing the same.
2. Description of Related Art
An alternating bielectrode conductivity meter has been known in the prior art as an instrument suitable for the measurement of the concentration of a solution and the like. This conductivity meter comprises two electrodes provided in a sensor portion. The electrodes are immersed in the solution to be measured, and an alternating voltage is applied between the electrodes to determine a value of resistance of the solution, by measuring a conductivity, which is a reciprocal of the value of resistance of the solution.
In a measurement of conductivity according to an alternative bielectrode method, charges are separated on a boundary surface of the metallic electrode and the solution. Thus, in the case where a condenser capacity (polarization capacity) is created in a solution having a high concentration, a polarization phenomenon can create a remarkably high error and can reduce the accuracy of the measurement of conductivity. To counter this effect, a conventional conductivity meter has resorted to platinum (Pt) as electrodes, and has subjected Pt electrodes to a platinum black plating.
It has been known that an equivalent circuit of electrodes is composed of an RC series circuit, and it is necessary to increase a loss coefficient D expressed by the following equation in order to reduce the polarization phenomenon of electrodes (D.infin..fwdarw.the polarized resistance ingredient=0). EQU D=2.pi.f.multidot.C.multidot.R
wherein f is a frequency applied from the instrument, C is a condenser portion on the boundary surface of the electrodes and the solution, and R is an interelectrode resistance.
However, in order to increase the loss coefficient D on the basis of the above-described relationship, it is necessary to increase either the frequency f, the condenser portion C, or the interelectrode resistance R. A cell constant of the electrodes is usually unchanged, so that an increase of the interelectrode resistance R is limited, and an increase of the frequency f is also limited. Thus, it is necessary for the condenser portion C to be capable of increasing a surface area of the electrodes. The above-described conventional conductivity meter has been constructed by paying attention to this point.
Thus, it has been known that if Pt is subjected to the platinum black plating, the condenser portion C is increased and the surface area of the electrodes can be increased by hundreds of time to that which existed before the platinum black plating. In short, the polarization phenomenon can be reduced by increasing the loss coefficient D.
However, a disadvantage occurs in that Pt is not only difficult to process, but it is also expensive and, as a result, the conductivity meter becomes expensive. In addition, when Pt is subjected to platinum black plating, usually Pt particles are deposited on a mirror-like surface of the electrodes without any presurface treatment. Thus, a problem can occur in that the gilt formed by the platinum black plating is apt to be separated during the operation and cannot provide a long-term use. Furthermore, a problem has also occurred in that lead wires, such as copper wires, are difficult to connect with the Pt electrodes. Thus, the prior art is still seeking a relatively inexpensive electrode with high measurement accuracy for a conductivity meter.